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<title>Debugging with GDB: Value History</title>

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<a name="Value-History"></a>
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<a name="Value-History-1"></a>
<h3 class="section">10.10 Value History</h3>

<a name="index-value-history"></a>
<a name="index-history-of-values-printed-by-GDB"></a>
<p>Values printed by the <code>print</code> command are saved in the <small>GDB</small>
<em>value history</em>.  This allows you to refer to them in other expressions.
Values are kept until the symbol table is re-read or discarded
(for example with the <code>file</code> or <code>symbol-file</code> commands).
When the symbol table changes, the value history is discarded,
since the values may contain pointers back to the types defined in the
symbol table.
</p>
<a name="index-_0024"></a>
<a name="index-_0024_0024"></a>
<a name="index-history-number"></a>
<p>The values printed are given <em>history numbers</em> by which you can
refer to them.  These are successive integers starting with one.
<code>print</code> shows you the history number assigned to a value by
printing &lsquo;<samp>$<var>num</var> = </samp>&rsquo; before the value; here <var>num</var> is the
history number.
</p>
<p>To refer to any previous value, use &lsquo;<samp>$</samp>&rsquo; followed by the value&rsquo;s
history number.  The way <code>print</code> labels its output is designed to
remind you of this.  Just <code>$</code> refers to the most recent value in
the history, and <code>$$</code> refers to the value before that.
<code>$$<var>n</var></code> refers to the <var>n</var>th value from the end; <code>$$2</code>
is the value just prior to <code>$$</code>, <code>$$1</code> is equivalent to
<code>$$</code>, and <code>$$0</code> is equivalent to <code>$</code>.
</p>
<p>For example, suppose you have just printed a pointer to a structure and
want to see the contents of the structure.  It suffices to type
</p>
<div class="smallexample">
<pre class="smallexample">p *$
</pre></div>

<p>If you have a chain of structures where the component <code>next</code> points
to the next one, you can print the contents of the next one with this:
</p>
<div class="smallexample">
<pre class="smallexample">p *$.next
</pre></div>

<p>You can print successive links in the chain by repeating this
command&mdash;which you can do by just typing <tt class="key">RET</tt>.
</p>
<p>Note that the history records values, not expressions.  If the value of
<code>x</code> is 4 and you type these commands:
</p>
<div class="smallexample">
<pre class="smallexample">print x
set x=5
</pre></div>

<p>then the value recorded in the value history by the <code>print</code> command
remains 4 even though the value of <code>x</code> has changed.
</p>
<dl compact="compact">
<dd><a name="index-show-values"></a>
</dd>
<dt><code>show values</code></dt>
<dd><p>Print the last ten values in the value history, with their item numbers.
This is like &lsquo;<samp>p&nbsp;$$9</samp>&rsquo; repeated ten times, except that <code>show
values</code> does not change the history.
</p>
</dd>
<dt><code>show values <var>n</var></code></dt>
<dd><p>Print ten history values centered on history item number <var>n</var>.
</p>
</dd>
<dt><code>show values +</code></dt>
<dd><p>Print ten history values just after the values last printed.  If no more
values are available, <code>show values +</code> produces no display.
</p></dd>
</dl>

<p>Pressing <tt class="key">RET</tt> to repeat <code>show values <var>n</var></code> has exactly the
same effect as &lsquo;<samp>show values +</samp>&rsquo;.
</p>
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